ICP Mass Spectrometry

Tuesday, 04 May 2010 00:00

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Introduction

ICP-MS which stands for inductively coupled plasma mass spectrometry is a type of mass spectrometry that is the fastest growing trace element technique available today.

Its main characteristic is its low detection limit in the sub parts per trillion (ppt) range and also offers the ability to quantify the test sample at the high parts per million (ppm) level. This instrument presents a hybrid of two separate instruments, inductively coupled plasma used to produce ions and a mass spectrometer used to separate and detect those ions. ICP-MS is also known as trace metal technique, which in comparison to ETA, FAA and ICP-OES, is more attractive technique for determination of metals present in traces.

Basic principle of operation

There are various number of ICP-MS designs available today, that differ in their ion-focusing system, mass separation device, design of interface and vacuum chamber. However, the basic principle of operation is quite the same in every different type of ICP-MS instrument.

The first step in analysis using this instrument is the introduction of the sample. Usually the sample must be in liquid form, which allows it to be pumped at 1ml/min with a peristaltic pump into a nebulizer. There it is converted into a fine aerosol with argon gas at about 1L/min. Next in the spray chamber, a separation of fine droplets from the larger ones, takes place followed by transporting the fine aerosol, from the exit tube of the spray chamber into the plasma torch, via the sample injector. [1]

The inductively coupled plasma (ICP) in the plasma torch is formed in a unique way. The plasma torch is consisted of three concentric tubes made of quartz. The end of this torch is placed inside an induction coil, which is supplied with radio-frequency electric current. Between the two outermost tubes, a 15 L/min flow of argon gas is introduced. In order to introduce free electrons into the gas stream, a high-voltage electric spark is applied for a short time. Furthermore, these electrons under the influence of the radio-frequency magnetic field are accelerated in different directions. Next, these electrons collide with argon atoms, sometimes causing the argon atoms to part with one of its electrons. This electron is further accelerated by the magnetic field, a process which continues until the rate of new electrons in collisions is balanced by the rate of recombination of electrons with argon ions. As a result, a very-high temperature (10.000 K) plasma discharge is formed at the open end of the tube. [2][3]

The ICP can be successfully retained in the quartz torch with the first flow of argon gas between the two outmost tubes, which keeps the plasma away from the walls of the torch. The second and the third flow of argon gas are introduced between the central tube and the intermediate tube, and into the central tube of the torch respectively. The liquid sample is introduced into the central channel, with the help of a nebulizer. [2]

The ICP in the ICP-MS instrument is positioned horizontally. Huge effort has been made in order to stop the process of generation of photons and accomplish the process of generation of positively charged ions. In fact, photons are the ones to blame if any increase of signal noise is to occur. The detection of large quantities of ions makes ICP-MS able to make low-ppt detection.

Exiting the plasma torch we come to the so called interface region. This part of the instrument is the most critical area of any ICP-MS. The interface region is settled between the plasma torch which is at 760 torr of atmospheric pressure, and the mass spectrometer analyzer region with its 10-6 torr of atmospheric pressure. The ions are transported efficiently, thanks to the vacuum of 1-2 torr present in the interface region, maintained by the roughing pump. The interface region consists of two nickel cones, named sampler and skimmer, each in possession of a small orifice (0,6-1,2 mm). The orifices allow the pass of ions through to the ion optics, where are further guided into the separation device. [4][5]

After leaving the interface region, the ions are directed into the main vacuum chamber by a series of electrostatic lens, named ion optics. The main function of this ion optic region is to electrostatically focus the ion beam to the mass separation device, therefore stopping the photons, particulates and neutral species to reach the detector, which would ultimately lead to signal noise.

The mass separation device is kept at vacuum of about 10-6 torr, and it makes the heart of the mass spectrometer. There are many different mass separation devices, but these three are the most commonly used: time-of-flight, quadrupole and magnetic sector technology.

The end goal of this process is to convert the analyte ions into an electrical signal. This is achieved with the help of an ion detector. Discrete dynode detector, as most commonly used, is consisted of series of metal dynodes placed along the whole length of the detector. The analyte ions pass through the mass filter and then impact on the first dynode, where they are converted into electrons which are therefore attracted to the next dynode. This leads to the process of electron multiplication, resulting in high stream of electrons coming from the final dynode. Finally, the data handling system converts the electron signal into data that reflects the analyte concentration in comparison to ICP-MS calibration standards. This electron multiplication process allows the detection system to process analyte sample in quantity range from ppt up to a few hundred ppms. [6]

The enormous interest of many scientists in this technique has lead ICP-MS instrument manufacturers to open very active Research and development programs, in order to improve instrument’s performance, applicability, usability and flexibility.